1. Field of the Invention
The present invention relates to a transmission circuit used for communication devices such as mobile phones and wireless LAN devices. The present invention particularly relates to a transmission circuit, which is small in size, operates with high efficiency and outputs a transmission signal having high linearity, and to a communication device using the transmission circuit.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to, regardless of a magnitude of output power thereof, secure precision of a transmission signal and operate with low power consumption. For such a communication device, a transmission circuit, which is small in size, operates with high efficiency and outputs a transmission signal having high linearity, is used. Hereinafter, conventional transmission circuits will be described.
One of the conventional transmission circuits is, for example, a transmission circuit disclosed in FIG. 1 of the Japanese Laid-Open Patent Publication No. 2004-048703 (hereinafter, referred to as Patent Document 1). FIG. 22 is a block diagram showing an exemplary structure of a conventional transmission circuit 700 disclosed in Patent Document 1. As shown in FIG. 22, the conventional transmission circuit 700 comprises a signal generation section 71, delta-sigma modulation section 72, angle modulation section 73, multiplying section 74, band-pass filter 75, output terminal 76 and a variable gain amplifier section 77.
In the conventional transmission circuit 700, the signal generation section 71 generates an amplitude signal and a phase signal. The amplitude signal is inputted to the delta-sigma modulation section 72. The delta-sigma modulation section 72 delta-sigma modulates the inputted amplitude signal, and outputs a resultant signal as a delta-sigma modulated signal. The delta-sigma modulated signal is inputted to the variable gain amplifier section 77. The variable gain amplifier section 77 amplifies the delta-sigma modulated signal by a gain corresponding to power information indicating a magnitude of an output power of the transmission circuit. The delta-sigma modulated signal amplified by the variable gain amplifier section 77 is inputted to the multiplying section 74.
The phase signal is inputted to the angle modulation section 73. The angle modulation section 73 angle-modulates the phase signal, and outputs a resultant signal as an angle-modulated signal. The angle-modulated signal is inputted to the multiplying section 74. The multiplying section 74 multiplies the delta-sigma modulated signal by the angle-modulated signal, and outputs a resultant signal as a modulation signal. The band-pass filter 75 removes quantization noise contained in the modulation signal. The modulation signal from which the quantization noise has been removed by the band-pass filter 75 is outputted as a transmission signal from the output terminal 76.
Another one of the conventional circuits is, for example, a transmission circuit disclosed in FIGS. 11 and 17 of Japanese Laid-Open Patent Publication No. 2004-072734 (hereinafter, referred to as Patent Document 2). FIG. 23A is a block diagram showing an exemplary structure of a conventional transmission circuit 800 disclosed in Patent Document 2. In FIG. 23A, the conventional transmission circuit 800 comprises a data generation section 81, vector modulation section 82, amplifier section 83, band-pass filter 84 and an antenna 85.
In the conventional transmission circuit 800, the data generation section 81 generates first data and second data which are orthogonal to each other. The data generation section 81 will be described later in detail. The vector modulation section 82 vector-modulates the first and second data, and outputs a resultant signal as a modulation signal. The amplifier section 83 amplifies the modulation signal. The band-pass filter 84 removes quantization noise contained in the modulation signal. The modulation signal whose quantization noise has been removed by the band-pass filter 84 is outputted from the antenna 85 as a transmission signal.
FIG. 23B is a block diagram showing an exemplary structure of the data generation section 81. As shown in FIG. 23B, the data generation section 81 includes a source data generation section 811, delta-sigma modulation section 812, multiplying section 813 and a multiplying section 814. The source data generation section 811 generates, based on a baseband signal, an amplitude signal, a standardized I signal and a standardized Q signal. The standardized I signal is calculated by dividing an I signal (in-phase signal), which is a baseband signal, by an amplitude signal. Similarly, the standardized Q signal is calculated by dividing a Q signal (quadrature-phase signal), which is a baseband signal, by the amplitude signal.
The amplitude signal is inputted to the delta-sigma modulation section 812. The delta-sigma modulation section 812 delta-sigma modulates the inputted amplitude signal, and outputs a resultant signal as a delta-sigma modulated signal. The multiplying section 813 multiplies the standardized I signal by the delta-sigma modulated signal, and outputs a signal resulting from the multiplication as the first data. The multiplying section 814 multiplies the standardized Q signal by the delta-sigma modulated signal, and outputs a signal resulting from the multiplication as the second data.
Still another one of the conventional transmission circuits is, for example, a transmission circuit disclosed in FIG. 24A of Japanese Laid-Open Patent Publication No. 2004-159319 (hereinafter, referred to as Patent Document 3). FIG. 24A is a block diagram showing an exemplary structure of a conventional transmission circuit 900 disclosed in Patent Document 3. As shown in FIG. 24A, the conventional transmission circuit 900 comprises a data generation section 91, data conversion section 92, modulation section 93, amplifier section 94, band-pass filter 95 and an antenna 96.
In the conventional transmission circuit 900, the data generation section 91 generates an I signal and a Q signal. The I and Q signals are inputted to the data conversion section 92. The data conversion section 92 converts the inputted I and Q signals into quantized signals each having a binary value. The data conversion section 92 will be described later in detail. The modulation section 93 modulates the quantized signals, and outputs a resultant signal as a modulation signal. The amplifier section 94 amplifies the modulation signal. The band-pass filter 95 removes quantization noise contained in the modulation signal amplified by the amplifier section 94. The modulation signal, whose quantization noise has been removed by the band-pass filter 95, is outputted from the antenna 96 as a transmission signal.
FIG. 24B is a block diagram showing an exemplary structure of the data conversion section 92. As shown in FIG. 24B, the data conversion section 92 includes a vector subtraction section 921, vector integration section 922 and vector quantization section 923. The I and Q signals are inputted to the data conversion section 92 from the data generation section 91. The I and Q signals are inputted to the vector integration section 922 via the vector subtraction section 921. The vector integration section 922 integrates the I and Q signals, respectively. Signals outputted from the vector integration section 922 are inputted to the vector quantization section 923. The vector quantization section 923 quantizes the inputted signals, and outputs resultant signals as quantized signals each having a binary value. Then, the vector subtraction section 921 subtracts the quantized signals from the inputted I and Q signals, respectively, thereby stabilizing an output of the quantized signals.
However, in the conventional transmission circuit 700 shown in FIG. 22, an envelope of the amplitude signal generated by the signal generation section 71 significantly varies, and thus quantization noise frequently occurs in the delta-sigma modulation section 72. For this reason, the band-pass filter 75 is required to have a steep characteristic in order to remove the quantization noise. This consequently causes an increase in power consumption and size of the transmission circuit. Thus, the conventional transmission circuit 700 has a problem of decreased power efficiency and increased circuit size. Also, there is a possibility that a quality of the transmission signal deteriorates due to the quantization noise which has not been removed by the band-pass filter 75.
Further, in the conventional transmission circuit 800 shown in FIGS. 23A and 23B, an envelope of the amplitude signal generated by the source data generation section 811 significantly varies, and thus quantization noise frequently occurs in the delta-sigma modulation section 812. For this reason, the band-pass filter 84 is required to have a steep characteristic in order to remove the quantization noise. This consequently causes an increase in power consumption and size of the transmission circuit. Thus, the conventional transmission circuit 800 has a problem of decreased power efficiency and increased circuit size. Also, there is a possibility that a quality of the transmission signal deteriorates due to the quantization noise which has not been removed by the band-pass filter 84.
Still further, in the conventional transmission circuit 900 shown in FIGS. 24A and 24B, an envelope of each signal inputted to the vector quantization section 923 significantly varies, and thus quantization noise frequently occurs in the vector quantization section 923. For this reason, the band-pass filter 95 is required to have a steep characteristic in order to remove the quantization noise. This consequently causes an increase in power consumption and size of the transmission circuit. Thus, the conventional transmission circuit 900 has a problem of decreased power efficiency and increased circuit size. Also, there is a possibility that a quality of the transmission signal deteriorates due to the quantization noise which has not been removed by the band-pass filter 95.